The acoustic mufflers are widely used to attenuate the noise transmitted in gas lines and they are particularly employed in compressors to attenuate the pressure transients generated by the opening of the suction and discharge valves of said compressor. In the refrigeration system, these pressure transients give rise to noise in different ways: sound radiation of the compressor due to the excitations of the shell resonances, usually from 2.5 kHz to 10 kHz; sound radiation due to the excitations of the cavity, usually from 300 Hz to 1 kHz; and sound radiation of the refrigeration appliance of the refrigeration system to which the compressor is coupled, due to the excitations of the components of this refrigeration system, mainly resulting from the low frequency pulses up to 2 kHz.
The suction acoustic muffler has several functions that are important for the good operation of the compressor, such as: gas direction, attenuation of the noise generated by the pulses resulting from suction, thermal insulation of the refrigerant gas drawn to the inside of the cylinder, and control of the suction valve dynamics. The suction acoustic mufflers have a major influence in the energetic efficiency of the compressor, due to the thermal insulation of the gas, load loss and valve operational coupling.
Besides the suction acoustic mufflers, the compressors of the refrigeration systems may be also provided, in the discharge thereof, with an acoustic dampening system, usually in the form of an acoustic muffler placed in the gas discharge line of the compressor and which conducts the gas compressed in the interior of the cylinder to a refrigeration system to which the compressor is usually associated.
The acoustic mufflers presently used are basically a combination of the resistive and reactive types, consisting of a sequence of volumes (usually one, two or three volumes in series, also known as expansion chambers) interconnected by gas ducts that conduct the refrigerant gas coming from the suction line directly to the suction valve, said gas ducts being generally open in the two ends thereof for the passage of the refrigerant gas. The acoustic mufflers are formed by gas ducts and volumes (FIGS. 2, 3 and 13) usually made of a solid material (plastic or metallic).
The gas displacement produces pulses, generating noises which are propagated in an opposite direction to that of the gas being displaced to the suction valve (FIG. 2). The smaller said pulses the more efficient the suction acoustic muffler at its acoustic outlet, through which the gas is admitted inside the acoustic muffler.
Its influence on the performance of the compressor is highly important and the dimensioning of the internal volumes and the length of the gas ducts of the suction muffler determines, to a great extent, the efficiency of the latter.
The related literature is rich in examples and applications of acoustic mufflers. (Hansen, H. “Engineering Noise Control”, 2003, Spon Press; Lyon, R. H., “Machinery Noise and Diagnostics”, 1987, Butterworth Publishers; Munjal, M. L. “Acoustics of Ducts and Mufflers”, 1987, New York Wiley-Interscience; Hamilton, J. F. “Measurement and Control of Compressor Noise”, 1988, Office of Publications, Purdue University, West Lafayette).
While widely used, the known suction acoustic mufflers of the volume-tube type have the disadvantage of presenting noise peaks in the acoustic modes typical of these tubes and volumes.
These acoustic mufflers present great attenuation in low frequencies (400 Hz to 800 Hz). However, in high frequencies, they lose performance due to the acoustic resonances of the elements in the form of tubes and volumes, generating more noise in the compressors. This behavior is much more intense in the acoustic mufflers of one volume. In general, the increases in the acoustic performance are achieved by increasing the volume or by reducing the diameters of the tubes, which is not always possible.
There are found applications of Helmholtz resonators, consisting of one tube and one volume which, although also attenuating the frequencies in which they are syntonized, have larger dimensions and increase the manufacturing complexity of the acoustic mufflers. Due to the larger size, the utilization of an arrangement of several Helmholtz resonators is unfeasible and its application is restricted to the attenuation of few frequencies.
One of the known techniques to attenuate the noise provoked by the passage of gas through acoustic mufflers is the dissipative technique, which uses fibrous material for constructing the acoustic muffler, in order to dissipate energy. Also known is the reactive technique, in which during wave propagation, a difference of impedance in a given frequency is generated.
However, the known acoustic muffler constructions with resonant reactive attenuation have the disadvantage of acting only in one frequency or in a narrow frequency band around the main frequency. Moreover, as a function of the constructive differences between the compressor and the acoustic muffler, the actuation of the latter in the expected frequency is not always the same, and a variation of about 100 Hz can occur above or below the desired frequency value to be attenuated.
There are known constructions of acoustic mufflers of the reactive type, comprising a plurality of resonators disposed along the extension of tube portions of acoustic mufflers (JP11093637A2), particularly in an arrangement of resonators radially projecting from the respective tube portion.
FIG. 4 illustrates a prior art construction for a resonator arrangement in which a gas duct of an acoustic muffler, not illustrated, comprises a plurality of resonant ducts distributed along the longitudinal extension of the respective gas duct, radially projecting therefrom.
While this solution minimizes the noise produced by the passage of gas through the respective acoustic muffler, it cannot be applied to acoustic mufflers of small refrigeration compressors, due to the large dimensions of said resonators and to the large volume occupied by them in the interior of the dampening chambers of said acoustic mufflers.